Copper etching is a complex oxidation-reduction process, wherein copper is converted from the metallic state into the ionic one and the oxidizing agent is reduced. The choice of an etching solution depends on many factors: type of the employed resistor coating applied to the circuit of printed boards, width of conductors and distance therebetween, possibility of repeated use of the etching solution, its production cost, and the like. For the manufacture of printed circuit boards by the subtractive method, acid solutions based on iron chloride and copper chloride are most extensively employed which have been hitherto employed only for a single time. This resulted in a high rate of consumption of chemicals for etching of printed circuit boards and was one of the principal sources of pollution of the environment.
Regeneration of spent etching solutions makes it possible to considerably reduce the rate of consumption of chemical agents for etching, bring down the amount and degree of contamination of waste water, improve the quality of printed circuit boards and increase the productivity of etching equipment. Regeneration can be effected by both chemical and electrochemical methods.
A method for chemical regeneration of spent copper chloride solutions by treatment thereof with hydrogen peroxide and hydrochloric acid, whereupon oxidation of ions of monovalent copper (the etching product in a copper-chloride solution) to the bivalent state occurs, thus resulting in a higher concentration of the oxidizing agent in solution. To maintain the required concentration of copper chloride in the etching solution, the latter is diluted with water, thus considerably increasing its volume. Excess etching solution is removed from the production cycle and subjected to further utilization.
This process features a great amount of waste water polluting the environment and is of a periodic character.
Also known are processes for electrochemical regeneration of spent chloride etching solutions, wherein metallic copper is recovered on a cathode in a powder-like form, while on an anode reduction of the oxidizing agent (iron chloride or copper chloride takes place). These processes are compatible with processes of etching of copper printed circuit boards in a single production cycle, which enables cutting down the rate of consumption of chemicals for etching, reducing the volume and degree of contamination of waste water, increasing the productivity of the etching equipment and a reduction of the cost of production of printed circuit boards.
A process for an electrochemical regeneration of iron-chloride solution (cf. U.S. Pat. No. 2,748,071), which is fed first into the cathodic space of a diaphragm electrolyzer is known in the art, wherein on a continuous steel band serving as a cathode, there occurs deposition of metallic copper as a powder and reduction of ions of trivalent iron unreacted in the course of etching. Then the solution is passed into the anodic space, wherein on graphite anodes ions of bivalent iron are oxidized to the trivalent state.
Performing the process under potentiostatic conditions makes it possible to eliminate the formation of gaseous chlorine on the anode, since the potential of graphite anodes corresponds to the potential of conversion of bivalent iron ions into trivalent iron ions. However, under these conditions the rate of deposition of copper on the cathode is insignificant thus lowering productivity of the unit for regeneration of iron-chloride solutions. The recovery of copper from the etching solutions as a powder contemplates the use of a device for its removal from the cathode, thus additionally complicating the regeneration equipment.
A process is known for regeneration of copper-chloride etching solutions (cf. U.S. Pat. No. 2,964,453), wherein such solution is regenerated in an electrolyzer formed from a steel tank lined with rubber and having 22 stationary anodes made from graphite and 54 rod-like copper cathodes secured on a hydraulic moving mechanism. Half of the cathodes are working members, while the other half are in a vessel for the removal of the copper deposit. The total surface area of the cathode is 12 square feet, while the surface area of the graphite anodes is six times that of the cathodes. The electrolysis is carried out under galvanostatic conditions at a current of 12,600 A. In this process metallic copper is recovered in powder-like form on copper rod-like cathodes while oxidation of ions of monovalent copper to the bivalent state occurs on the graphite anodes.
The use of high current densities and a partial self-regeneration of the copper-chloride etching solution due to oxidation of ions of monovalent copper with air oxygen results in that on the graphite anodes, in addition to oxidation of ions of monovalent copper, there occurs liberation of gaseous chlorine. The evolved chlorine is removed from the electrolyzer and delivered to scrubbers for absorption by an alkali solution.
This process is characterized by a high productivity relative to the recovered copper due to the use of high current densities on the electrodes; however, withdrawal of gaseous chlorine from the process cycle changes the etching solution composition and requires correction by using hydrochloric acid.
The liberation of gaseous chlorine on the anodes and formation of powdered copper complicates the equipment employed for the regeneration process.
A process is known for the electrochemical regeneration of spent copper-iron chloride etching solution (cf. USSR Inventor's Certificate No. 548051), which is carried out in a diaphragm electrolyzer under a current density ranging from 8 to 20 A/dm.sup.2 on the electrodes. This solution is fed into the cathodic space of the electrolyzer, wherein on titanium cathodes recovery of powdered metallic copper occurs and ions of trivalent iron are reduced.
The recovery of metallic powder-like copper on the cathodes requires additional devices for periodic cleaning of the cathodes and washing of copper powder. During the operation of the regeneration unit water in the copper washing system becomes contaminated with the etching solution components and is an additional source of pollution of the environment.
From the cathodic space the copper-iron chloride etching solution is passed into the anodic space of the electrolyzer, wherein on graphite anodes processes of oxidation of ions of bivalent iron and recovery of gaseous chlorine occur. The gaseous chlorine liberated on the anodes is partly absorbed in the electrolyzer due to chemical interaction with ions of bivalent iron according to the scheme: EQU 2Fe.sup.2+ +Cl.sub.2 .fwdarw.2Fe.sup.3+ +2Cl.sup.-
The unreacted chlorine from the electrolyzer is passed into a vessel with the etching solution, wherein it is fully absorbed due to the reaction.
For a better absorption of gaseous chlorine in the electrolyzer, the etching solution is passed into the anodic space in counter-current relative to the liberated chlorine at a rate at least 1.5 times higher than that of the stream in the cathodic space. The provision of different rates of stream in the cathodic and anodic spaces of the electrolyzer requires the use of a partitioning diaphragm which causes an increase of the voltage impressed to the electrolyzer and a higher rate of consumption of electric power for the recovery of copper.
This process has a high productivity relative to the recovered copper and a constant composition of the etching solution due to recycling of chlorine evolved on the anode. However, evolution of gaseous chlorine on the anode requires thorough sealing of the entire system, presence of transfer conduits for chlorine and special absorbents, which complicate the process equipment and do not prevent release of chlorine into the atmosphere in case the system becomes untight.
Regeneration of the spent etching solution is effected at current densities of up to 20 A/dm.sup.2, and further intensification of the process is impossible due to a sharp increase of the amount of gaseous chlorine evolved on the anode, which substantially hinders its absorption.